1. Field of the Invention
Strapping machines for binding articles are well known and generally perform the following operation in sequence, feeding strap into a loop about an article, clamping the free end of strap, withdrawing strap to tension the loop about the article, sealing overlapped portions of the strap together and shearing the loop from the remaining strap. A machine for performing the above sequence of operations is shown in U.S. Pat. No. 3,232,217.
2. Description of the Prior Art
One of the difficulties encountered with strapping machines is the failure to stop the strap in precisely the same location upon repetitive feeding cycles so that the exact same length of overlap is obtained on each cycle. If insufficient overlap is obtained, the strap may not be properly sealed and will come undone. Excessive overlap on the other hand may catch in the machine preventing ejection of the bound article, or at least cause a waste of strap material and a potential safety hazard from an exposed strap end. The machine shown in U.S. Pat. No. 3,906,851 provides a sensor located upstream from the sealing assembly where the strap overlap is formed and a positive stop in the area of the sealing assembly. This combination depends upon a controlled delayed termination of the strap feed to assure that the strap engages the stop. Slight variations in strap feed speed or in the extent of the delay cause varying degrees of strap overlap.
Another difficulty with prior machines has been that the complicated clamping mechanisms for holding the free end of the strap while the loop is tensioned require frequent repairs. Generally a pair of clamping arms are mounted on pivotable linkages and driven by a separate hydraulic cylinder as shown in U.S. Pat. Nos. 3,232,217 and 3,687,059. Such arrangements require undue maintenance and adjustment due to the large number of parts involved.
Finally, the shearing mechanisms of strapping machines used for feeding steel strap wear with extended use and must be adjusted or replaced. Although this does not usually require a complete disassembly of the machine it is often difficult to perform due to location of the shear and its means of adjusting and holding in place. Since shear wear occurs in proportion to amount of use the result is frequent and extended downtime.